linux-kernel - Re: [PATCH 02/14] x86/alternatives: Add nested alternatives macros

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Date: Fri, 31 May 2024 17:10:08 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Borislav Petkov <bp@...nel.org>, X86 ML <x86@...nel.org>,
 LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 02/14] x86/alternatives: Add nested alternatives macros



Borislav Petkov je 31. 05. 24 ob 16:36 napisal:
> On Fri, May 31, 2024 at 04:30:31PM +0200, Uros Bizjak wrote:
>> Please don't resurrect the %P modifier, use %c instead.
> 
> Btw, out of curiosity, is %P being phased out?

No ;)

> I'm looking at
> 
> 41cd2e1ee96e ("x86/asm: Use %c/%n instead of %P operand modifier in asm templates")
> 
> and yeah, %c is the generic one while %P is the x86-specific one but
> phasing latter out is probably going to take a bunch of gcc releases...

The intention of '%P' operand modifier is primarily to be used with PIC, 
where:

'P'        If used for a function, print the PLT
            suffix and generate PIC code.  For
            example, emit 'foo@...' instead of 'foo'
            for the function foo().  If used for a
            constant, drop all syntax-specific
            prefixes and issue the bare constant.  See
            'p' above.

the fact that is handles constants is due to historic reasons.

The '%c' operand modifier will also check its operand that it is indeed 
an immediate integer operand, so should be used with 'i' operand constraint:

'c'        Require a constant operand and print the
            constant expression with no punctuation.

Uros.